Many welding applications such as MIG (metal inert gas) or GMAW (gas metal arc welding) utilize a wire feeder to provide filler metal to the weld. Generally, the wire feeder will provide wire at a nominally constant speed. A typical prior art wire feeder includes a motor that pulls wire from a reel and feeds the wire at a wire feed speed to the weld arc. The motor is controlled by a wire feed controller that may be a stand alone controller or may be part of a controller that controls other aspects of the welding process. The wire feed controller controls the speed of the wire feeder and typically includes a potentiometer (or digital up/down input buttons) on a front panel of the controller which the user uses to set wire feed speed.
A trigger on the gun (torch) is pulled when the user wants to weld. A trigger circuit causes power to be provided to the wire feed motor, and wire is fed to the arc, along with welding power. When the user releases the trigger, power is removed from the wire feed motor and the arc. Under normal operating conditions the wire feeder provides the wire to the arc and the current draw of the motor is within an acceptable range.
However, occasionally a feed problem such as the wire inadvertently being welded to the gun tip, or becoming tangled, will cause the wire feed motor to stall. The stalled motor will draw excessive current, and cause overheating of the motor windings. This can damage the motor, or cause other problems.
One known way to prevent motor damage from excessive current draw due to a stall is to provide a fuse or fusible link electrically between the motor and power source. When excessive current is drawn, the fuse opens the motor power line. However, the fuse or fusible link needs to be replaced prior to restarting the wire feeder, causing inconvenience and down-time.
One known protection device is a thermistor, which has been used in non-welding applications. However, many non-welding thermistor applications involve using the thermistor to control current through a relay coil, and opening the coil in response to undesired high current. This sort of scheme requires an additional relay, and may result in excessive wear and tear to the relay.
Other non-welding thermistor applications involve using the thermistor as both a protective element and a control element, wherein the thermistor is used to inhibit current under extreme conditions, and controls the magnitude of power provided under normal conditions. Such a scheme is of little use for an application such as a wire feed motor having the power controlled elsewhere.
One thermistor application involves using a thermistor for a start-up circuit protection. The thermistor is shunted with a relay, and the relay is closed after the start-up circuit has precharged components, and the power source is connected to the proper input power, and the thermistor's protective function ends. If the power source is connected to improper input power the thermistor blocks the pre-charge, and the relay is not closed. However, in an application such as a wire feed motor the excessive current may occur at times other than start-up.
Accordingly, it is desirable to have a protective circuit for a welding wire feed motor that is relatively inexpensive, unlikely to wear, useful beyond start-up, and is not used to otherwise control power. Preferably, such a circuit should not require user intervention to restart the motor after the protective function is performed.